NEW YORK (MCT) – Three people were killed in the East Harlem explosion that leveled two buildings Wednesday, and the search continues for nine occupants who are unaccounted for, city officials said.

Word of an additional death came shortly after the National Transportation Safety Board said investigators will focus on Con Edison's response to the gas leak that caused the blast.

"We want to find out not only what happened, but we want to find out why it happened ... to make sure something like this never happens again," said NTSB board member Robert Sumwalt, shortly after a team of investigators arrived at the scene Wednesday evening.

Those investigators will look at any possible issues with the distribution pipeline that delivers natural gas to the five-story buildings and how Con Edison handled the situation, Sumwalt said.

"We will be looking at Con Edison ... their integrity management system, seeing how they handled complaints ... oversight of Con Edison by federal and state officials," Sumwalt said.

Investigators will look at Con Edison's call log and at any evidence of third-party damage by digging, he said.

Con Edison said the utility is "going to be cooperating with all agencies."

The explosion injured dozens, hurling debris onto the elevated Metro-North tracks along Park Avenue, shattering windows in the neighborhood and sending people running into the streets as a plume of smoke rose above the skyline.

Con Edison workers were on their way to address the leak, but "the explosion occurred before that team could arrive," Mayor Bill de Blasio said.

"This is a tragedy of the worst kind because there was no indication in time to save people," the mayor said. "From what we know now, the only indication of danger came about 15 minutes earlier, when a gas leak was reported to Con Edison."

Among the victims, two have what are considered critical/serious, life-threatening injuries; five have serious, but not life-threatening, injuries; and 20 have minor injuries, the FDNY said.

The FDNY said its count reflected the number of victims the department treated. City hospitals, meanwhile, reported treating 61 patients, some of whom were walk-ins. Most had minor injuries. Two FBI agents were among the injured, although those injuries were not life-threatening, according to the FBI.

About 250 members of the FDNY were conducting a "thorough search" of the rubble, de Blasio said, adding that he expected it to be "a long operation." Those attempting to reach family or loved ones among the nine missing are urged to call 311, the mayor's office said.

Michael Parrella, a FDNY spokesman, said the bulk of the fire that destroyed the buildings at 1644 and 1646 Park Ave. had been extinguished by Wednesday evening and firefighters were tackling hot spots among the debris. Parrella said firefighters also were simultaneously searching the debris.

Some parts of the debris pile were not accessible "due to a sinkhole that has developed in front of the buildings due to a subsurface water main break, likely closed by the explosion," Parrella said.

Among those missing were Andreas Panagopoulos. There has been no sign of Panagopoulos at three hospitals, leaving his family with the dreadful feeling that he might be buried in the rubble.

"We don't know anything," Tulio Gomez, his brother-in-law, said as he waited Wednesday afternoon at a makeshift command center a block from the scene.

Panagopoulos, 42, last spoke to his wife, Lisbeth Perez, when she called him at 8:30 a.m., about 45 minutes before the explosion.

"They talked in the morning, and he went back to sleep," said Gomez, 43, of Queens.

He said his brother-in-law runs his advertising company from home, a second-floor apartment at 1646 Park Ave.

"He usually doesn't leave," Gomez said.

The National Transportation Safety Board said it "has launched a go-team" to investigate the explosion.

A report of a gas odor from a resident at 1652 Park Ave. came at 9:13 a.m., Con Edison said in a tweet. Con Edison crews were dispatched at 9:15 a.m. "and arrived just after explosion," the utility said in the tweet.

The first call reporting the explosion to 911 was at 9:31 a.m. and the first firefighting unit was on the scene at 9:33 a.m., said FDNY Commissioner Salvatore Cassano.

Officials said there were six residential units in one building and nine in the other that were destroyed by the explosion.

Multiple utility crews responded to the scene after the explosion, checking gas lines, "making the area safe and working closely with the FDNY," said Elizabeth Matthews, a Con Ed spokeswoman.

On Tuesday, Ruben Borrero, 32, who has lived at 1646 Park Ave. his entire life, said he and his family complained to the landlord about a gas smell in the building.

"We had to air out our apartment yesterday and we called 311 and they told us to file a complaint," he said.

Borrero heard about the collapse from a friend and returned home to find his residence in rubble.

"All I saw was an empty lot," he said in an interview at an American Red Cross shelter at PS 57 at Third Avenue and East 115 Street. "My life was in that building, my father's ashes and my family dog."

Borrero's mother, Sarah, was at work, and his sister, Kimberly, 16, was at school during the collapse. Borrero had left in the morning to drop his son off with a baby sitter.

He said his family, who lived on the second floor, will stay with relatives.

Borrero struggled to comprehend losing everything.

"All we have is what we have on our backs," he said, adding, "I need to see what the next step is. I never lost everything. Thank God I have a lot of family and friends who I can turn to. We are starting now from scratch, from nothing. I will make sure that I will be there for my family and, amen, we are alive."

Area hospitals reported that patients suffered mostly minor injuries.

Dr. Kevin Chason, director of emergency management for Mount Sinai Health System, said 22 patients were taken to Mount Sinai _ about half were walk-ins, and the rest came in by ambulance. He said there was one serious injury _ a woman with head trauma who was found under the rubble. She was in stable but critical condition, he said.

"Mostly they have minor injuries _ cuts, scrapes, lacerations, broken bones, some inhalation injuries from the smoke and debris in the area," said Chason, director of emergency management for Mount Sinai Health System.

A hospital official said Wednesday evening that 19 of the 22 patients had been discharged.

As Chason spoke, six people had already been treated and released.

Seventeen patients have been treated at Metropolitan Hospital Center, with nine adults and one pediatric patient remaining "under evaluation and in stable condition," said Noel Alicea, associate director of public affairs. "Six came by ambulance and 11 walked in on their own with minor complaints," Alicea said.

Harlem Hospital Center reported 13 patients, three of them children, one of whom is in critical condition and two in stable condition, said spokesman Kencle Satchell. The 10 adults, ranging in age from 20 to 79 years old, were in stable condition, Satchell said. Six of the patients were walk-ins.

A spokeswoman for NewYork-Presbyterian Weill Cornell Medical Center Wednesday night said nine victims were being evaluated at the hospital. No other information was available.

Metro-North Railroad crews used construction equipment, including a crane capable of lifting 1,200 pounds, and a large trash bin to remove debris from the tracks after the explosion.

The debris was cleared from the tracks and all service to and from Grand Central Terminal was restored during the early evening rush after an earlier suspension.